Why are my tests negative?

“I feel like I’ve got a urinary tract infection… but my doctor says I can’t because my tests are negative”

Many chronic urinary tract infection (UTI) sufferers have had their symptoms dismissed by their GP or urologist on the basis of dipstick tests and mid-stream urine (MSU) cultures. Worryingly, these tests have been shown to miss many infections. Guidelines from the National Institute for Health and Care Excellence (NICE) – the body that produces treatment guidelines for England and Wales – insist that a positive dipstick and MSU are needed to diagnose a urinary tract infection in patients who have not got better after initial antibiotics.

Just because your tests do not find evidence of an infection, this is not evidence that you don’t have a UTI

How do doctors test for urinary tract infection?

Dipstick tests and MSUs are the standard tests that both GPs and specialists use to diagnose UTIs.

Dipstick tests measure signs of bacterial infection including the number of white blood cells in urine and compounds called nitrites produced by bacteria. White bloods cells and nitrites are known as ‘bacterial infection markers’

MSUs identify which types of bacteria are present in the urine and in what numbers by growing them in a petri dish in a laboratory.

Why do dipstick tests and MSUs fail to find urinary tract infections?

There are several practical reasons why dipstick tests and MSUs can miss the presence of infection. Any of these factors can give a negative test result:

Antibiotics that partly clear an infection will reduce the levels of bacteria in your urine

Drinking a lot to try to flush out an infection will dilute your urine

Bacteria that have moved into the cells of the bladder wall – which happens in chronic UTI – are not detected by these tests

But there are also serious problems with the way the tests were designed and how they are interpreted.

What’s wrong with dipstick tests and MSUs?

Diagnostic criteria for UTI is based on outdated research

The use of MSUs and dipstick tests to diagnose UTIs are based on research by a scientist called Kass from the 1950s based on a study of a small number of pregnant women suffering from severe infections of the kidneys.

These women, who were not suffering from lower urinary tract infections, are not representative of the typical UTI sufferer.

According to Kass, there have to be at least 105 (100,000) bacteria per millilitre of urine for an infection to be present. But infection does not suddenly happen when a threshold is crossed. Research has shown that a UTI can be present when there are much lower numbers of bacteria in the urine.

Kass’s criteria for bacterial infection in MSUs was to achieve strong growth of a single type of bacteria. It assumes that the normal bladder is sterile – for example does not contain bacteria.

A result of ‘weak mixed growth’ was thought to show the sample had been contaminated. The presence of cells from the bladder wall (epithelial cells) was also thought to show contamination and these samples were rejected.

But new research says the opposite…

Studies show that:

A healthy bladder is not sterile. Just like the gut, the bladder has its own balance of good and bad bacteria called the ‘urinary biome’. Our bladder contains about 500 different types of bacteria which usually coexist happily.

There is no evidence that UTIs are caused by a single type of bacteria and multiple studies suggest that multi bacterial infections are common. A healthy bladder has a balance of hundreds of different strains of bacteria which are more or less sensitive to different types of antibiotics. An antibiotic that wipes out all or most of one type of bacteria can allow numbers of another strain to rocket. The second strain of bacteria, which might not have been a problem when there was just a few of them, now becomes an infection.

Some strains of bacteria are easier to culture – they grow easily in the laboratory. Other strains divide too slowly to show up in the usual length of time given and they can be ‘crowded out’ by stronger, fast-growing types. Some types of bacteria die in the presence of oxygen so cannot be grown in a laboratory.

Research shows that epithelial cells are found in greater numbers in the urine of chronic UTI sufferers. The wall of the bladder and urethra is made up of urothelial cells about five layers deep. As an infection advances, bacteria move from the urine into the cells.

Finally the body’s immune system responds to the infected cells by shedding them. At this point, bacteria that have been living inside the cell burst back out into the urine and the infection flares up again.

Relying on inaccurate tests sees many thousands of UTI sufferers misdiagnosed or even dismissed as ‘problem patients’

What is the most reliable way to diagnose chronic UTI?

The most useful thing that doctors can do is to listen to their patients and ask them detailed questions about their symptoms and how they started.

Tests for bacteria or pyuria (white blood cells) do not establish the diagnosis of UTI

The diagnosis of UTI is primarily based on symptoms and signs.

Research suggests that microscopy – analysing fresh urine under a microscope using a haemocytometer (an device used to count white blood cells and epithelial cells) – combined with listening to a patient’s story is the best way to diagnose UTI.